This invention relates to improvements in methods and materials for laminating printed circuits at low pressures on flat bed presses.
In the manufacture of flexible and flat cable circuits, layers of high dielectric strength plastic films with low shrinkage and good stability at high temperatures are first combined with copper or other conductive metal by an additive or substractive process and electrical circuits are produced by etching or deposition of these metals on the plastic surface. These circuits are then covered by a layer of similar adhesive coated plastic dielectric film to protect and insulate the finished circuit. Holes may be punched or drilled in the layers to allow for connections to the finished circuit.
The circuit assembly is then laid up with a protective cap in single or multiple layers (up to 20 plus) and then laminated under heat (ambient - 400.degree. F.) and pressure (250 to 400 p.s.i.) taking extreme care to register the circuits and drilled holes. This process is normally performed in a flat bed laminating press by the operator of the press, using pad materials consisting of multiple layers piled on between the steel caul plates and the finished circuit. Typically, the first layer, next to the steel plate, is a non-sticking release material such as a tetrafluoroethylene coated fiberglass cloth. The second layer includes multiple plies of kraft paper or equivalent cellulosic wood product; the third layer consists of one or more plies of a thermoplastic film; and the final layer consists of one or more layers of release materials such as tetrafluoroethylene or polyvinyl fluoride.
The resultant lamelliform pad may include 10 or more loosely piled layers of dissimilar materials, each having two sides presenting up to 20 opportunities for possible contamination. Because each layer is customarily handcut and laid up by hand, there is a high margin for error in the sequencing of dissimilar materials, resulting in defective circuits.
The pads are arranged in a prescribed order on either side of the circuit assembly with the release layer facing the assembly. Multiple circuit assemblies with associated pads are placed in the press to form a book, and the entire book is pressed in one operation to laminate the individual circuit assemblies. When the laminating process is complete the pads are stripped away and the laminated circuit assemblies are removed for further handling.
Among the problems found in the prior art process were erratic results and lamination failures due to errors in selecting and laying up the multiple plies of dissimilar materials, hot spots appearing in the laminate creases, pockets of gas and other contamination trapped in the pad, lateral shifting of the pad materials and the circuit laminate, and moisture absorption by the cellulosic pad.
Accordingly, it is an object of this invention to provide a stratiform sheet, for use in laminating printed circuits, characterized by freedom from included gases, contaminants, and creases.
It is also an object of this invention to provide such a stratiform sheet that achieves substantially more reliable method of preparing the book for the laminating step, and more dependable results.
It is further an object of this invention to provide such a stratiform sheet that achieves more even and rapid heat distribution with uniform pressure to a printed circuit.
It is an additional object of this invention to provide a means for minimizing lateral movement of the components of the book in the laminating process and to provide means and methods of reducing the incidence of rejection of the laminated circuit product due to lamination failures.
Yet another object of this invention is to reduce operator error and increase lamination cycle efficiency.